The present invention relates to a process for manufacturing a (U,Pu)O2 mixed powder from non-free-flowing UO2 powders.
The manufacture of fuel for light-water reactors, based on uranium and plutonium oxides, generally called MOX fuel, has been the subject of various developments associated with the desire to recycle plutonium recovered during spent fuel reprocessing.
The manufacture and irradiation of MOX fuel in light-water reactors are now considered to be a solution for providing reasonable resistance to the proliferation of plutonium, present in a form separated from the fission products, whether this plutonium is either of civilian or military origin.
Several processes for manufacturing MOX fuel have been developed over the last two decades, some of the processes involving the complete milling of the UO2 and PuO2 powders in order to provide an intimate blend, while others are limited to milling only a fraction of these powders.
The MIMAS (standing for MIcronization and MASter blend) process, which was developed by the Applicant of the present invention (see FIG. 1), comprises the micronization, by milling, of only a fraction of the final blend and uses two successive blending operations to achieve isotopic homogenization and to take advantage of the use of free-flowing UO2 incoming products (especially to ensure that the dies of the presses used for pelletizing are properly filled). Using free-flowing UO2 powders in the second blending operation and limiting the milling to only the first blending operation simplify the manufacture (for example by dispensing with the operations of precompacting/granulating or spheroidization of the mixed oxide blend) and have greatly simplified, at the start of industrial implementation, the qualification of MOX fuel by users and the licensing process by the Nuclear Safety Authorities (thanks to the similarity in behavior between this MOX fuel and UO2 fuel).
Various versions of the MIMAS process have been applied, sometimes under names different from MIMAS, but all characterized by two successive blending operations, the second of which uses free-flowing UO2.
UO2 which serves as feed material in the manufacture of enriched-uranium fuel and, in the great majority of cases, in the manufacture of MOX fuel, is obtained by the conversion of uranium hexafluoride. There are industrial conversion processes which produce free-flowing UO2 powder. This is especially the case with two industrial conversion processes using a wet route, known in the art by the respective names xe2x80x9cAUCxe2x80x9d, coming from the intermediate product (Ammonium Uranyl Carbonate), and xe2x80x9cTU2xe2x80x9d, coming from the uranium transformation unit in which the conversion is carried out. One of the drawbacks of these wet conversion processes is the production of a large amount of liquid effluents which have to be treated before discharge. The wet conversion processes, some of which do not produce free-flowing UO2, are gradually being replaced with dry processes which allow the gaseous effluents to be recycled but which generally produce non-free-flowing UO2 powder.
For the purpose of diversifying the sources of UO2 powder for manufacturing MOX fuel by MIMAS-type processes, it is therefore useful to be able to employ non-free-flowing UO2 powders.
Non-free-flowing UO2 powder conditioning processes, for transforming it into free-flowing UO2 granules, and therefore having properties suitable for feeding a pelletizing press, are known. Various mechanical granulation processes, such as precompaction-granulation or agglomeration-spheroidization, have been developed and are used on an industrial scale in UO2 fuel manufacturing plants.
Experience has shown that these granulation processes produce granules of insufficient mechanical strength for correct implementation of the second blending operation which characterizes the MIMAS processes and similar processes. Under the optimum operation of the second blender, the granules are damaged and the flowability of the secondary blend is impaired: the fuel pellets which result therefrom suffer from major defects (excessive variability in the physical properties of the product, local differential shrinkage defects, etc.). Alternatively, if the method of operating the second blender is modified so as to achieve gentle mixing of the powders to be blended, or if the apparatus used for the second blending is modified for the same purpose, the uniformity of distribution of the plutonium within the fuel may be impaired and the MOX pellets thus produced no longer meet the maximum plutonium content variability criteria.
To avoid the abovementioned drawbacks, the process for manufacturing MOX fuel from non-free-flowing UO2 powder, which is the subject matter of the invention, comprises a mechanical granulation treatment of the non-free-flowing UO2 powder, which does not modify the chemical properties (such as a stoichiometry) and morphological properties (such as the particle size) of the UO2 powder, but which does nevertheless ensure the mechanical strength and flowability that are required to successfully carry out the second blending operation and the pelletizing operation, respectively.
The invention thus obviates the need to supply the MIMAS-type processes with free flowing UO2 powders as feed materials.
According to one advantageous method of implementing the invention, non-free-flowing UO2 powder is used, one part of which is used, as it is, for incorporation in the first blend and one part of which undergoes a granulation treatment before being incorporated into the second blend.
In a variant, as a nonlimiting example, said granulation treatment may also be applied to the non-free-flowing UO2 fraction fed in the first blend
In order to avoid the drawback of the abovementioned lack of mechanical strength of UO2 granulated by one of the usual conditioning processes, the mechanical treatment according to the invention is carried out either by forcing the non-free-flowing UO2 powder through a screen or sieve, or by compressing this powder into tablets under a high pressure, as required for obtaining suitable non-friability properties, and then crushing said tablets. When necessary, one or more binders and/or lubricants may be added beforehand to the UO2 powder.